In the fields of various electronic devices such as semiconductor devices, for example, which need microprocessing procedures, because of requirements for further enlargement of device density and integration, the pattern size has to be miniaturized more and more. One of the semiconductor manufacturing processes which plays an important role for formation of an extraordinarily fine pattern is a photolithographic process.
Most of current photolithographic processes are currently carried out on the basis of reduction projection exposure. The resolution thereof is restricted by diffraction limits of light, and generally a spatial resolution of only about one-third of the wavelength of a light source is obtainable. In consideration of this, the wavelength for exposure has been shortened such as, for example, by using a KrF excimer laser or an ArF excimer laser as an exposure light source. As a result of this, microprocessing of about 100 nm order has currently been enabled.
Along with the introduction of shorter wavelengths, various modifications have been attempted in relation to photomasks, for example, used there because the light intercepting property is insufficient where conventional masks are used.
An example is found in Japanese Laid-Open Patent Application, Publication No. H06-095363 and Japanese Laid-Open Patent Application, Publication No. H06-095358, wherein a photomask having a light blocking film made of silicon is proposed, for, with a conventional chromium-based photomask, the light intercepting property with respect to KrF excimer lasers (wavelength 248 nm) is insufficient.
However, the photomasks disclosed in these patent documents are anyway those masks to be used in the reduction projection exposure process.
In the photolithography wherein the wavelength of used light sources has been shortened more and more as described above, in addition to the matter of photomasks, there are many problems such as bulkiness of apparatus, development of lenses usable in shortened wavelengths, cost of apparatus, cost of usable resist materials, and so on.
On the other hand, as another attempt to enabling optical microprocessing with a resolution finer than the wavelength of light, a method that uses near-field light has been proposed.
Such near-field optical lithography is free from the restriction due to the diffraction limits of light, and thus a spatial resolution of one-third or less of the light source wavelength is attainable. Another advantage is that, if a mercury lamp or a semiconductor laser is used as an exposure light source, the exposure light source can be made very small and therefore the apparatus can be made very compact and yet the cost of the apparatus can be made lower.
U.S. Pat. No. 6,171,730 shows an example of lithography using near-field light.
This patent document discloses a method in which a near-field exposure mask provided with a light blocking film having openings narrower than the light source wavelength is closely contacted to a resist with a small clearance less than 100 nm (i.e. near field region), and a fine pattern formed on the mask is transferred to the resist by one-shot exposure.
In the aforementioned U.S. Pat. No. 6,171,730 wherein the exposure is carried out while the near-field exposure mask is closely contacted to the resist, a portion of near-field light produced at the openings of the light blocking layer would be transformed into propagating light. Even if the incident light is being perpendicularly incident on the light blocking layer, since the propagating light produced by transformation from the near-field light has low directivity, it would be propagated in oblique directions. This propagating light is propagated through the object material to be exposed, while it is reflected by the substrate surface and then by the light blocking layer (i.e. multiple reflection). As a result of propagation of light through the object material to be exposed while being reflected repeatedly, it is possible that the propagating light reaches an adjacent pattern area to adversely affect the quality of the pattern. Furthermore, depending on a resist used, the adhesion to the light blocking layer of the mask may be strong so that the mask may be broken upon removal thereof or the resist may be peeled off from the substrate.
Additionally, if a used resist material is of the type that development contrast is produced by reaction that uses, as a catalyst, acid being produced by the exposure, such as, for example, a chemical amplification type resist or an optical cationic polymerization type resist, the light blocking layer of the mask would be corroded by the produced acid and hence the lifetime of the mask would become shorter.